1. Technical Field
The present invention relates to an image forming apparatus including a charging device for charging an image forming carrier with a corona discharge generated by a corona wire, and particularly to a device for preventing pollution of a charging device by forming an airflow across the charging device.
2. Background Art
In an image forming apparatus such as, for example, printer, copier, facsimile machine or the like using an electrophotographic process, an electrostatic latent image formed on a photosensitive drum is developed by charging a surface of the photosensitive drum with a corona discharge and then applying toner on the photosensitive drum. The developed image is then transferred and fixed onto a recording medium such as a cut sheet of paper.
Generally, as a unit for charging the surface of the photosensitive drum, a corona discharging wire (i.e., charging wire) is used for the production of an uniform electric field in response to application of high voltage to charge the surface of the photosensitive drum with high potential for enabling the photosensitive drum to attract toner and thereby form the latent image. Such charging device not only discharges harmful ozone which causes image blurring and shading due to the degradation of the surface of the photosensitive drum, but also causes undesirable adhesion of fine dust particles to the corona wire because of the high electric field formed on the surface of the charging wire whenever the corona wire generates a high discharge current. The fine dust particles typically include flying toner which is the residue of toner left on the surface of the photosensitive drum after an image transfer operation that has not been cleaned by a cleaning unit, dust entered into the image forming apparatus, and paper powder flaking off the recording paper. Therefore, the surface of the corona wire becomes contaminated and this contamination of the corona wire tends to produce an uneven discharge that results in an unevenness in the developed image. Hence, it is necessary to frequency remove and manually clean the corona wire as well as the inside of the charging device.
Conventional designs for cleaning mechanisms such as represented, for example, by U.S. Pat. No. 5,392,099 for Image Forming Apparatus Having Cleaning Member For Cleaning Charging Wire issued to Kusumoto et al., endeavor to devise a cleaning pad arranged to slide along the corona wire in a main scanning direction in order to clean the corona wire. This cleaning technique however requires a costly cleaning mechanism constructed within the charging device.
Other designs prefer to use a contact type of charging unit instead of a corona discharge wire for the charging operation. For example, in U.S. Pat. No. 5,081,496 for Image Forming Apparatus Having A Ventilated Contact Charging Unit issued to Takeda employs a detachable process cartridge in which a charging roller as a contact charging member is disposed to charge the surface of the photosensitive drum in lieu of the corona discharge wire. Recognizing, however, that a contact charging roller tends to create an accumulation of discharge products at the side where the charging roller contacts the photosensitive drum, Takeda '496 proposes a ventilation system in a detachable process cartridge in which air flow is channeled to flow along a direction of a contact portion established between the contact charging roller and the photosensitive drum so as to remove the discharge products formed around the discharging roller.
While the conventional air ventilation system used in an image forming apparatus to avoid contamination may have differences in structure and functions such as, for example, U.S. Pat. No. 5,189,473 issued to Negoro et al., for removing toner particles leaking from a discharged toner container, and U.S. Pat. No. 5,390,006 for Imaging Forming Apparatus With Improved Exhaust Flow issued Wakabayashi et al., for handling toner and other debris between a fixing device and the photosensitive drum, it is our observation that no image forming device has been constructed to solve these particular problems related to the corona discharge wire. We have found that due to the high voltage applied to the corona wire and the concomitant high operating temperature of the corona wire, the corona wire is extremely susceptible to deleterious chemical reactions in the presence of ions accompanying the residual toner and other debris. Moreover, it is also our observation that no image forming device has been constructed in a manner in which the cost of the air ventilation system can be minimized while facilitating accessibility of the interior of image forming apparatus.